1. Field of the Invention
The present invention relates to an ignition timing control system for internal combustion engines which serves a function of detecting the occurence of knocking from the vibration, sound or the like produced inside and outside of the cylinders of an engine due to the cylinder pressure so as to retard the ignition timing in response to the occurrence of knock signals.
2. Description of the Prior Art
The ignition timing of an engine must be controlled in accordance with the engine conditions so as to ensure that optimum operation of the engine. Ignition timing control systems known in the art are generally so designed that the ignition timing is controlled in accordance with the engine conditions represented by the engine speed detected by a centrifugal advance mechanism and the intake negative pressure detected by a vacuum advance mechanism.
It is known in the art that generally the best way from the standpoint of engine efficiency and fuel consumption is to effect the ignition at a position near a so-called minimum advance for best torque or MBT, and the ignition timing must be controlled to the MBT in accordance with the engine conditions.
On the other hand, if the ignition timing is advanced gradually under certain engine conditions, eventually knocking will be caused and stable operation of the engine will be made impossible. Generally the relationship between the MBT and amount of ignition spark advance that causes knocking is such that the knocking limit is reached before the MBT under low speed and load conditions. Also the knocking limit is subject to the effects of atmospheric conditions, such as, air temperature and humidity and consequently in accordance with the existing ignition timing control systems the desired spark advance is programmed according to such parameters as the engine speed and intake negative pressure so as to be retarded considerably with respect to the MBT and thereby to prevent the occurrence of knock throughout the range of engine operating conditions. As a result, the power output and fuel consumption of an engine are kept below the rated performance of the engine.
It is well known in the art that there is a close relation between the ignition timing and the cylinder pressure so that when a mixture is exploded, no harmonic component (frequency component usually in the range of 5 to 10 KH.sub.Z) will be superimposed on the cylinder pressure when there is no knocking but a harmonic pressure variation will be caused when there is knocking.
This effect results in the generation of vibration or sound outside cylinder. Thus, many different types of so-called knocking feedback ignition systems have been investigated in which such vibration or sound is detected to control the ignition timing.
In accordance with these known systems, the ignition timing is controlled in response to every combustion phenomenon in such a manner that the presence or absence of knocking is detected on every explosion stroke of the respective cylinders and the ignition timing is controlled in accordance with the resulting signal. As for example, the ignition timing is retarded a predetermined angle when there is knocking and the ignition timing is advanced a predetermined angle when it is determined that there is no knocking.
A great disadvantage of applying the feedback in this type of known system is that when the key switch is turned off, the desired amount of spark advance or retard for the ignition timing corresponding to the knock condition will be erased with the result that when the engine is started again, the ignition timing cannot be controlled in consideration of knocking and knocking will be caused.